I think it is a good idea to ride the horse a couple of times before you buy it.

The reason I say this is because, about 10 yrs ago my dad had a friend that owned a quarter horse, he was a very friendly horse, BUT he was very picky about who could ride him. He broke the owners wife's leg, and a friend of the family daughter's arm when they tried riding him. His name was Rusty and he liked to buck alot when someone was on his back.

I was able to ride him after ALOT of convincing on my dad's part that I wouldn't get hurt.
I started out just going and brushing him, or using the lunge line with him. After a few months of being around this hourse I finally decided I should try and mount him.
He did try to buck me off at first, until he realized I wasn't going anywhere.
We then got along really well, (Although there was the one time he bit me on the shoulder because I was talking to someone else and not paying attention to him.
Just a little story to show, not all horses will accept other riders on them. Just remember there is a horse for you, and I hope this one is it.
Good Luck, I hope you enjoy him.
P.S. Just to let you know, RUSTY ended up dying because he decided to roll in the field one morning and ended up twisting his stomach inside out, if that makes sens e!!!
Poor thing